-LRB- CNN -RRB- -- As the world knows , peace can sometimes be fleeting .

One-time basketball bad boy Ron Artest , who changed his name to Metta World Peace and said it was meaningful and inspirational , was ejected Sunday from the Los Angeles Lakers-Oklahoma City Thunder game for hitting James Harden in the head with his elbow .

After scoring against the Thunder in the second quarter , World Peace was cheering his own shot when he hit the Oklahoma City guard in the back of the head , knocking him down .

Watch the NBA.com video of the hit

Harden was on the floor for several minutes , and did not return to the game after being diagnosed with a concussion , according to the Thunder .

World Peace was ejected from the game and faces possible suspension . The Lakers won 114-106 over the Thunder in double overtime .

`` During that play I just dunked on -LRB- Kevin Durant -RRB- and -LRB- Serge -RRB- Ibaka , and I got really emotional and excited . It was unfortunate that James had to get hit with an unintentional elbow , '' World Peace told reporters after the game .

`` I hope he 's OK . The Thunder , they 're playing for a championship this year , so I hope that he 's OK and I apologize to the Thunder and to James Harden , '' he said .

Hours later , World Peace tweeted that he watched the replay again : `` Oooo . . My celebration of the dunk really was too much ... Did n't even see James ... Omg ... Looks bad . ''

Harden told ABC 's Lisa Salters he had `` a little bit of a headache . ''

On Monday , the Thunder said Harden was undergoing testing .

`` Harden participated in a series of limited activities per NBA guidelines , but has additional steps that must be taken under the league-mandated concussion policy before he can make his return to the court , '' the team statement said . `` He will be re-evaluated tomorrow and is currently listed as day-to-day . ''

World Peace 's act was called `` disgraceful '' by game commentators , and sports analysts said the behavior was reminiscent of the ball player they once knew as Ron Artest .

`` He has gone to such lengths to rehabilitate his image , and to revert back to this ? He lost control , '' said Michael Wilbon , an ESPN analyst .

Sports fans were baffled , some even amused , when World Peace announced last year that he planned to legally change his name from Ron Artest .

`` Ron Artest has contemplated the name change for years and always knew that he wanted his last name to be World Peace . But it took many years of research and soul searching to find a first name that was both personally meaningful and inspirational , '' his publicist said at the time .

Back when World Peace was still Artest and playing for the Indiana Pacers , he made headlines in 2004 for his role in a brawl between players and fans at a Pacers-Pistons game after somebody threw a drink on him .

The NBA suspended Artest for 86 games .

Fast forward to September 2011 , when Artest announced the name change .

Even before it was finalized by the court , World Peace was working on making the world a more peaceful place .

In 2011 , he raffled off his 2010 NBA Championship ring to help mentally ill youths . For his work with the youth , he was given the NBA 's citizenship award for philanthropic work .

A star turn on `` Dancing With the Stars '' garnered the basketball player a new legion of fans .

But in the world , peace sometimes comes with a price : The NBA is now reviewing World Peace 's less than peaceable actions .

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NEW : The Oklahoma City Thunder says James Harden is undergoing testing

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World Peace of the Los Angeles Lakers knocks down Thunder 's Harden

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World Peace , formerly known as Ron Artest , is ejected from the Lakers-Thunder game

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Harden tells ABC he has `` a little bit of a headache ''